Trunked radio systems have developed as a means of increasing user mobile or portable units capacity in systems which operate in a fixed geographic area. In the past, a typical system was comprised of either a conventional mobile relay system or a single site trunked system.
A conventional mobile relay system may be comprised of a full duplex transceiver, receiving and transmitting the audio signal to a fleet of mobile or portable units operating on the reverse pairing of those frequencies. This provides extended unit to unit coverage within the coverage area of the mobile relay. The capacity of a mobile relay system is severely limited by the number of users and the frequency and length of particular calls.
Trunked systems alleviate the constraints of a single mobile relay system by operating on several alternative frequency pairs, and communications between units is done on channels which are dynamically assigned to them by a control unit on a separate control channel pair. In effect, trunking systems provide for automatic channel assignment in a mobile or portable radio. Trunked radio systems improve both spectral efficiency and operating characteristics of land mobile radio systems.
Coverage from a given radio site is limited by Federal Communications Commission rules and regulations which require reducing the effective radiated power output of the mobile relay or trunked transmitter as the height above the average terrain is increased. This, in conjunction with local terrain and buildings, results in limited coverage from a single site.
Increasing the amount of coverage may be accomplished by constructing additional trunked systems or mobile relays on additional sites to provide coverage in the desired areas. The selection of another trunked system or a mobile relay is an economic one, based on the coverage desired and the number of units that will be served by the coverage of the additional site(s). Additionally, the FCC has loading standards, measured in units per frequency pair, so that providing too many frequency pairs can put a system operator at risk of losing exclusive use of their frequency pairs.
The selection of single mobile relays to "fill in" trouble spots is the best selection but suffers from a loss of system discipline as the unit operator has been tasked with the manual selection of the correct trunked or mobile relay system as they travel through the territory covered by the combination of trunked systems and mobile relays.